Natural killer (NK) cells have been demonstrated to play a role in the control of viral infection before the establishment of a specific cytolytic T cell response mediated by CD3-positive MHC-restricted T cells. Recently, specific recognition of alloantigens by NK cells was also reported. Most of the cells displaying natural killer activity belong to the CD3-CD4-CD8-CD16+CD56+ subset of peripheral blood lymphocytes. The aim of this study is to define the mechanism of target cell recognition by NK cells. As a first step, we asked whether specificity could be demonstrated in the recognition of virus-infected cells by NK cells. A large panel of clones was derived from pure CD3- CD56+ cells of healthy donors, to test their ability to lyse virus-infected autologous cells. Surprisingly, only about half of the clones (41/86) were able to kill human Herpes virus 6 (HHV6)-infected autologous PHA-blasts, while all of them lysed the NK-sensitive cell line K562. A group of clones (17) was further characterized for its ability to recognize autologous or allogeneic infected cells, while another group (15) was analyzed to asses the lysis of cells infected with other viruses (EBV and HSV). The results indicated for the first time that cells infected with different viruses are recognized by different sets of clones. This implies specificity in the recognition by CD3-CD56+ lymphocytes. Furthermore, it suggests that NK cells play a role in the immune response against HHV6 and more generally against viral infection.